Microacoustic ultra-high frequency device

Abstract

Device that operates using acoustic waves and is implemented in a layer stack comprising the following: - a carrier (SU) - a decoupling layer (DCL) - a piezoelectric layer (PL) - an electrode structure (ES) for exciting acoustic longitudinal waves, wherein the decoupling layer acoustically decouples the layers above from the layers below the decoupling layer, so that acoustic coupling of volume waves with the carrier is reduced, wherein the decoupling layer has an elastic modulus of less than 1 GPa and a density of less than 500 kg / m3.

Description

[0001] The invention relates to a microacoustic device realized as a stack on a support substrate and based on a piezoelectric thin film. The device can be implemented as a SAW resonator.

[0002] The development of next-generation mobile communication systems requires devices with an outstanding combination of various performance criteria, such as high frequency, high quality factor, strong electromechanical coupling (k2), and a low temperature coefficient of frequency (TCF). Currently, commonly used SAW devices based on lithium tantalate (LiTaO3), lithium niobate (LiNbO3), or quartz bulk substrates are widely used in low- and mid-band LTE applications, such as the 1 GHz to 2.5 GHz range. However, future applications, such as 5G, require devices operating up to 6 GHz, which cannot yet be supported by these conventional SAW devices.

[0003] Initial experiments in this frequency range with SAW resonators are based on sandwich-like enclosed stacks, where a piezoelectric thin film is stacked onto a support substrate to achieve high coupling. Optionally, some dielectric materials can be added between the support substrate and the piezoelectric thin film. Their purpose is to optimize certain criteria, such as a low temperature coefficient of frequency. 5G frequency bands require the use of wave types with exceptionally high speeds, such as longitudinal surface acoustic waves. However, these waves are not confined to the surface and therefore tend to lose their wave energy into the substrate. Consequently, initial applications in the high-frequency range exhibit a poor quality factor.

[0004] Publication US 2015 / 0287905 A1 discloses a layer system comprising a substrate layer, a piezoelectric layer, and an aluminum nitride layer arranged in between.

[0005] An objective of the present invention is to provide a microacoustic device with an improved quality factor and reduced coupling of wave energy into a volume substrate, which can be used for a frequency range of up to 6 GHz.

[0006] These and other tasks are fulfilled by a device according to claim 1. Further specific features of the new device and advantageous embodiments can be found in the dependent claims.

[0007] The device is based on a stacked material system that includes a high-coupling piezoelectric thin film. The piezoelectric thin film comprises, for example, LiTaO3 or LiNbO3 with varying cut angles. The cut angle is chosen such that the main mode, due to its high speed of sound, is a longitudinal wave. Other modes are suppressed through design modifications and a suitable choice of cut angle.

[0008] The stack is formed on a support substrate. The piezoelectric thin film enables the excitation of a longitudinal wave as the main mode. Longitudinal waves are preferred over Raleigh waves or shear waves due to their high speed of sound. The coupling of wave energy into the bulk support is reduced by means of a decoupling layer placed between the piezoelectric thin film and the support.

[0009] The decoupling properties can be achieved with a material that has a low density and a low modulus of elasticity. The decoupling layer comprises a material with a modulus of elasticity of less than 1 GPa and a density of less than 500 kg / m³. 3 .

[0010] A material that can be produced with such properties is an aerogel.

[0011] An aerogel is a synthetic, porous, ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and low thermal conductivity. Colloquial names include "solid air" because an aerogel layer exhibits properties similar to an air gap. Aerogels can be made from a wide variety of chemical compounds. An aerogel structure results from sol-gel polymerization, meaning that monomers (simple molecules) react with other monomers to form a sol, or a substance composed of linked, cross-linked macromolecules with deposits of a liquid solution between them. When the material is critically heated, the liquid evaporates, leaving behind the linked, cross-linked macromolecular framework.

[0012] Aerogels can be based on metal oxides, such as aluminum oxide, chromium oxide, and tin oxide. Carbon aerogels are also known.

[0013] For the present application, silicon oxide aerogel is preferred. It is the most common type of aerogel. However, other aerogels can also be used for the decoupling layer.

[0014] Silicon oxide aerogel can be produced with a porosity of 80 to 99.8%. Therefore, it exhibits a density of 0.16 to 500 kg / m³. 3 Materials with such low density perfectly prevent acoustic waves from passing through a decoupling layer. Therefore, the wave energy cannot be transferred to a layer below this decoupling layer, resulting in a limitation of the longitudinal wave's acoustic energy within the piezoelectric thin film. Consequently, the quality factor of the new device is improved.

[0015] The device has an exceptionally high Q-factor (quality factor) and produces low losses even at high frequencies for longitudinal waves.

[0016] In a preferred embodiment, the acoustic wave-operating device is a SAW resonator. The interdigital converter of the SAW resonator has electrode fingers arranged with a grid spacing that is the distance between the geometric centers of two adjacent electrode fingers. Due to the longitudinal wave type with its exceptionally high speed, interdigital converters can still be produced using conventional geometries and standard techniques, even for waves with a grid-dependent resonant frequency in an ultra-high frequency range extending up to 6 GHz. The SAW device can be used to fabricate filter devices for mobile communications in complex standards such as 5G.

[0017] According to further embodiments, optional functional dielectric thin films can be added between a support substrate and a piezoelectric thin film to improve certain filter properties, such as TCF or suppression of volume waves.

[0018] The device can include a TCF compensation layer arranged between the decoupling layer and the piezoelectric layer. The TCF compensation layer comprises SiO2, doped SiO2, or GeO2.

[0019] A shielding layer can be provided between the substrate and decoupling layers. This layer serves to prevent the unwanted effect of an electric field that would otherwise occur at the interface between a silicon substrate or support and a SiO2 layer deposited directly above it. Materials suitable for the shielding layer include poly-Si, Si3N4, AlN, Al2O3, SiC, diamond-like carbon, and diamond. Furthermore, these materials exhibit a relatively high speed of sound, making them useful for wave confinement.

[0020] To provide a limitation of the wave energy, the substrate can be selected from, for example, Si, sapphire, SiC, graphene and diamond.

[0021] The invention is explained in more detail below with reference to an embodiment and the accompanying figures. The figures are drawn schematically and not to scale. Identical elements or elements with the same or a comparable function are designated by the same reference numerals. Fig. Figure 1 shows a schematic cross-section through a stacked device according to a first embodiment; Fig. Figure 2 shows a schematic cross-section through a stacked device according to a second embodiment; Fig. Figure 3 shows schematic electrode structures of a SAW resonator; Fig. Figure 4 shows a schematic block diagram of a filter consisting of resonators, as in Fig. 3 shown, built; Fig. Figure 5 shows the admittance of a resonator according to one embodiment and a reference example; Fig. Figure 6 shows the absolute value of the admittance of a resonator around the resonance frequency according to one embodiment and a reference example; Fig. Figure 7 shows the absolute value of the impedance of a resonator around the resonance frequency according to one embodiment and a reference example.

[0022] Fig. Figure 1 shows a schematic cross-section through a layer stack according to a SAW device of a first embodiment of the invention. A support substrate SU forms the base of the stack. A decoupling layer DCL is arranged above the support SU. A piezoelectric thin film PL is arranged on top of this. An electrode structure, which, for example, implements a SAW filter comprising resonators, is applied to the stack.

[0023] Fig. Figure 2 shows a schematic cross-section through a layer stack according to a SAW device of a second embodiment of the invention, which includes further optional layers. In comparison to the stack made of Fig. In Figure 1, two additional layers are introduced. A shielding layer, e.g., made of polysilicon, is positioned between the substrate SU and the decoupling layer DCL. The decoupling layer DCL can comprise an aerogel. The shielding layer can have a thickness of 0.2 µm to 2.5 µm.

[0024] Furthermore, a TCF compensation layer TCL made of approximately 100 nm to 800 nm SiO2 is arranged between the decoupling layer DCL and the piezoelectric thin film PL. The thin film PL consists of, for example, LN, which is applied with a cutting angle that supports longitudinal wave excitation and propagation.

[0025] The electrode structures ES consist of an aluminum-based metallization and include interdigital transducers. The grid spacing of the interdigital transducers is set to a value corresponding to the desired wavelength and is half a wavelength of the longitudinal wave propagated in the piezoelectric thin film PL. In a subsequent process step of device fabrication, a passivation layer of a dielectric or photoresist can be applied (not shown in the figure).

[0026] One specific embodiment comprises the following layers from bottom to top: a Si carrier SU, a shielding layer made of 500 nm poly-Si, a decoupling layer DCL made of approximately 25 nm to 75 nm silicon oxide aerogel, a piezoelectric thin film PL made of LN170Yrot90X or LN20 with a thickness of 100 nm to 500 nm, an Al-based electrode structure ES comprising Cu with a height of approximately 70 nm to 150 nm.

[0027] A schematic electrode structure ES of a SAW resonator R is shown in Fig. Figure 3 shows the resonator R having a well-known metallization structure consisting of an interdigital converter IDT arranged in an acoustic track between two reflectors REF.

[0028] Fig. Figure 4 shows a schematic block diagram of a branch-type filter that can be constructed from the new resonators. The filter comprises at least one series resonator RS in a series signal line and one parallel resonator RP in a shunt line extending from the series signal line to ground. A pair of these two resonators forms a basic section BS, which already exhibits filtering function. Real branch-type filters comprise n such basic sections BS, the number n of which depends on the desired degree of achievable filter selectivity. The figure shows three basic sections and one additional series resonator RS as examples.

[0029] Fig. Figure 5 shows the absolute value of the admittance of a resonator, as shown in Fig. 3 shown, and based on a layer stack according to Fig. Figure 3 is designated as Curve 1. For reference, Group 2 shows the admittance of a comparable stack lacking the decoupling layer DCL. In this embodiment, the resonators resonate at approximately 5 GHz. It exhibits a sufficiently high k2 for high-frequency applications but is slightly reduced compared to Curve 2. The real part demonstrates the significant improvement in loss achieved by adding a decoupling layer DCL, which decouples the acoustics in the piezoelectric thin film PL from the underlying layer stack. Additional volumetric waves, e.g., at 5 and 5.8 GHz, are visible, which can be suppressed by further optimization of the layer stack. The coupling factor k2 can be further optimized by adjusting the thickness and cut angle.

[0030] Apart from the improved resonance peaks of group 2, a disturbance mode that appears at about 5200 MHz in curve 2 has almost disappeared and has shifted to a lower frequency of about 5000 MHz, as can be seen from curve 1.

[0031] Fig. Figure 6 shows the improvement in loss during resonance and Fig. 7 at the antiresonance frequency of a single-port resonator based on a layer stack according to Fig. 2. Similar to in Fig. Figure 5 corresponds to curve 1 of the invention, while curve 2 corresponds to a respective reference example. The resonance frequency was plotted as an absolute value of the admittance |Y| and the antiresonance peak as an absolute value of the impedance |Z|. In both figures, the curves are scaled to the same frequencies for better comparison. It can be seen that the quality factor Q is clearly improved due to sharper and higher peaks. List of terms and reference symbols used 1, 2 curves assigned to the new resonator and the reference BS basic section of a branch type filter DCL decoupling layer ES electrode structure IDT Interdigital Converter PL piezoelectric thin film REF Reflector RP,RS,R Resonator SHL dielectric layer / shielding layer SU Carrier TCL TCF compensation layer

Claims

[1] Device which operates using acoustic waves and is implemented in a layer stack comprising the following: - a carrier (SU) - a decoupling layer (DCL) - a piezoelectric layer (PL) - an electrode structure (ES) for exciting acoustic longitudinal waves, wherein the decoupling layer acoustically decouples the layers above from the layers below the decoupling layer, so that acoustic coupling of volume waves with the carrier is reduced, wherein the decoupling layer has an elastic modulus of less than 1 GPa and a density of less than 500 kg / m3. [2] Device according to the preceding claim, wherein the decoupling layer comprises an aerogel. [3] Device according to the preceding claim, wherein the decoupling layer comprises a silicon oxide aerogel. [4] Device according to any one of the preceding claims, which is implemented as a SAW device, which includes an interdigital converter (IDT) as an electrode structure. [5] Device according to one of the preceding claims comprising a TCF compensation layer (TCL) arranged between the decoupling layer and the piezoelectric layer. [6] Device according to the preceding claim, wherein the TCF compensation layer comprises SiO2, doped SiO2 or GeO2. [7] Device according to one of the preceding claims comprising a shielding layer (SHL) arranged between the carrier and the decoupling layer. [8] Device according to the preceding claim, wherein the shielding layer comprises polysilicon, Si3N4, AlN, Al2O3, SiC, diamond-like carbon or diamond. [9] Device according to any one of the preceding claims, which is implemented as a SAW resonator (R), wherein the material of the piezoelectric film has a cutting angle that is chosen to support excitation and propagation of longitudinal waves, wherein the SAW resonator comprises an interdigital transducer as an electrode structure, where the grid spacing of the interdigital converter is set to correspond to a longitudinal wave and a resonant frequency between 2.5 GHz and 6 GHz.

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